This invention relates to a method of producing acrylonitrile copolymer having superior non-inflammability.
There are at least two well known methods of producing acrylonitrile polymers. One method employes solution polymerization in a homogeneous system with dimethyl sulfoxide or dimethyl formamide as a solvent for the copolymers of the acrylonitrile series. The other method employes emulsion polymerization in an aqueous system with an emulsifier such as any proper anionic surface active agent or cation surface active agent. These prior process have been used industrially.
However, the solution polymerization process has not yet been used for the production of acrylonitrile copolymer containing vinyl chloride which belongs to the so-called "modacrylic" fiber designation, having a small amount of acrylonitrile in the molecule.
The reason why the solution polymerization is not used for the production of such copolymer appears to be as follows. A chain transfer reaction between the solvent molecule and the free radical of the growing polymer molecule occurs too often, thereby resulting in a considerable decrease of mean degree of polymerization. Moreover, since the velocity of polymerization reaction is slow, it takes a long period for completion of the reaction, thereby resulting in the appearance of a deep color. These are disadvantages in the solution polymerization method of producing "modacrylic" fibers. Moreover, these disadvantages especially appear in the production of the copolymer of acrylonitrile series containing vinyl chloride.
Thus, for the production of acrylonitrile copolymer containing vinyl chloride, the emulsion polymerization has been ordinarily used.
If the emulsion polymerization process is compared with the solution polymerization process, there are apparent such advantages as the velocity of polymerization reaction of the former is larger than that of the latter and the degree of polymerization of the obtained polymer in the former is much greater than that in the latter. However, obtaining a spinning solution from the polymer obtained by using the emulsion polymerization is complex and difficult. After the polymerization process, the obtained polymer should be separated from the aqueous medium by salting out and filtration. After washing, granulation and drying the separated polymer, the spinning solution can be at last prepared by dissolving the dried polymer. This involved process is accompanied by a large expenditure of labor cost and investment cost. Moreover, use of a large amount of purified water is necessary. Such a method for the preparation of spinning solution can never be considered advantageous from the industrial and economic standpoints.
In order to overcome such deficiencies in the solution polymerization process and also in the emulsion polymerization process, technical studies have been carried out previously. For examples, the Japanese Patent Publication S. 49(1974)-14546, discloses a method of polymerization of a mixture of monomers containing 80 to 20 weight percent of acrylonitrile in a mixed solvent consisting of an organic solvent and water, especially, in a mixed solvent consisting of dimethyl formamide and water whose ratio of dimethyl formamide:water, is in the range of 60:40 to 85:15 by weight.
However, it is very difficult to carry out the production of acrylonitrile polymer containing vinyl chloride by the method mentioned above. The difficulty can be easily understood from the following. The above mentioned patent recommends use of sulfonated monomers as a third component for smooth polymerization reaction. Examples of such sulfonated monomers are sulfoalkyl esters of acrylic acid or methacrylic acid or their salts, styrene sulfonic acid or derivatives of styrene sulfonic acid or their salts, and methallyl sulfonic acid or its salts. However, in the case of polymerization of plural components system of acrylonitrile series containing vinyl chloride, using methallyl sulfonic acid or its salts in a mixed solvent whose ratio of dimethyl formamide to water is relatively small, the polymerization system becomes very unstable producing coagulation of the obtained polymerization reaction product or generating polymer precipitates and accumulations. Moreover, only a polymer of low degree of polymerization can be obtained. In order to improve this instability of the polymerization system, if the ratio of dimethyl formamide to water is increased, the resulting polymerization system becomes slightly more stable and the polymerization reaction barely proceeds in a stable manner. However, the degree of polymerization decreases considerably and the yield of polymer becomes low. Therefore, such a modification is not practical.
On the other hand, in case sulfoalkyl esters of acrylic acid or methacrylic acid or their salts are used as the third component, although the polymerization reaction can be carried out even in a mixed solvent whose ratio of dimethyl formamide to water is relatively small, polymer accumulations and precipitates appear within the polymer latex and it is difficult to obtain a stable and uniform latex. Furthermore, if the ratio of dimethyl formamide to water is increased when sulfoalkyl esters of acrylic acid or methacrylic acid or their salts are used as the third component, the tendency to generate polymer accumulations and precipitates in the polymer latex is improved, considerably. However, this modification also cannot be used practically because the degree of polymerization becomes too low and the yield of polymer is unsatisfactory. Thus, the method of the above Japanese Patent is difficult to apply to the polymerization of acrylonitrile series containing vinyl chloride.